1. Field of Invention
The present invention relates to a solar cell, and more particularly, to a solar cell with a superlattice structure and a fabricating method thereof.
2. Related Art
At present, the commercial solar cells according to the materials can be classified into the silicon and the III-V compound semiconductor solar cells. For a silicon-based solar cell, since silicon material is an indirect energy bandgap material, the light absorption of the silicon solar cell is poor and the energy conversion efficiency of the silicon-based solar cell is low, at most 24% or so at present. For a III-V compound semiconductor solar cell, since. the III-V group compound material has a direct energy bandgap, the Ill-V group compound semiconductor has advantages such as high energy conversion efficiency and strong anti-radiation ability, and it can work under high temperature and can be suitable for thin film growth. The quality of the III-V group compound semiconductor solar cells is superior to that of the silicon semiconductor solar cells. Therefore, many research reports indicate that the III-V group compound semiconductor material is more applicable to the development of high-performance solar cells.
As for a conventional III-V compound solar cell, in the fabricating process, common substrates for an epitaxy process are classified into GaAs substrates and Ge substrates. Since the lattice constant of the Ge substrate is approximately equal to that of GaAs, the Ge substrate can preferably match with various typical III-V compound materials, and can be fabricated a junction around 0.67eV. Thus the light with long-wavelength can be absorbed by the III-V group materials and the efficiency of the III-V group compound semiconductor solar cell is also improved. Therefore, the Ge is used as a common substrate in the market.
According to the structure, III-V group compound solar cells are classified into single-junction and multi-junction solar cells. The single-junction solar cell only absorbs the light of spectral region above single energy bandgap and the theoretical value of the conversion efficiency is 26-29% at most, and this is the main reason for the efficiency of the silicon-based solar cell cannot be further enhanced. Multi-junction solar cells, on the other hand, are usually made by a stack of semiconductors with different energy bandgap, to induce the spectral response covering the different energy regions of the solar spectrum, such as ultraviolet light, visible light, and infrared light. Therefore, the conversion efficiency of the multi-junction solar cell can be enhanced. The theoretical value of the conversion efficiency of a well-designed multi-junction solar cell can reach more than 50%.
As a result, the structures of components of solar cells are also gradually progressing. Firstly, the single-junction structure has been replaced by a multi-junction structure and the number of junctions has gradually increased. In recent years, a quantum well structure has appeared and gradually replaced the bulk structure, and thus a solar cell with high efficiency has been obtained. The structure of III-V compound solar cells and the fabricating method thereof have been claimed in many patents, such as U.S. Pat. Nos. 4,404,421, 4,128,733, 4,451,691, 5,944,913, 6,281,426, 6,372,980, 3,993,506, and 6,437,233.
At present, scores of different materials can be used to stack multi-junction solar cells. However, this does not mean that any materials can be stacked together due to lattice matching. When the difference among lattice constants of stacked materials is large, the strain will generate and to degrade the crystal quality. That will lead to a crystalline defect, and further decrease the conversion efficiency of the solar cell. The most efficient multi-junction solar cell in commerce to present is composed of three junctions, GaInP(1.85 eV)/GaAs(1.40 eV)/Ge(0.67 eV), i.e., a GaInP/GaAs/Ge triple junction solar cell. In theory, it can have a conversion efficiency of 40%. In the structure of the conventional triple junction solar cell, if the content of In of GaAs is added optionally to form a GaInAs compound, the energy bandgap thereof will be adjusted downward to the midst (1.26 eV) between 1.85eV and 0.67eV, so as to optimize the overall efficiency of the components. However, in fact, in order to decrease the difference between the lattice constants of the middle layer and the Ge substrate, the In content must be limited to within 1% so as to the energy bandgap cannot achieve 1.25 eV, and relatively the absorption of the infrared region is reduced. Thus, the efficiency of conventional triple junction solar cell cannot be optimized. In order to further enhance the efficiency, the U.S. Sandia National Laboratory recently provided a GaInP(1.85 eV)/GaAs(1.40 eV)/InGaNAs(1.0 eV)/Ge(0.67 eV) four-junction cell, i.e., InGaNAs is added between the GaAs junction and the Ge junction. Since the energy bandgap of InGaNAs can be controlled to be within 1.0 eV, the loss of absorption between 1.40 eV and 0.67 eV is compensated. Theoretically, the conversion efficiency can be greatly improved to more than 40%. However, in practice, since the quality of the InGaNAs material is rapidly deteriorated with the increase of the content of N, such that the carrier lifetime becomes shorter. Therefore, it is very difficult to obtain an InGaNAs (1.0eV) epitaxy material with high quality, such that the structure still cannot be brought into practice at present.
However, many III-V semiconductor materials have lattice constants matching with GaAs and Ge, such as AlGaAs, GaInAs, GaInAsN, GaInP, and GaAsN, but each kind of those materials must be carefully prepared to stack because not all arbitrary combinations can completely match. Therefore, the application region is small and the selection of materials for epitaxy growth is limited.
Furthermore, because it is difficult to match the lattices of conventional materials, stain is easily generated among the layers in the solar cell structure, such that a thicker epitaxy film cannot obtain, and the absorption region in the solar cell structure is limited, and further the efficiency of the solar cell is often negatively affected.
In recent years, the InGaNAs material is often used as an absorption material of 1.0 eV. But the crystal quality of the material is poor, resulting in disadvantages such as shorter carrier diffusion length, lower mobility, shorter lifetime, and higher impurities concentration. As a result, the efficiency of the whole solar cell cannot be enhanced efficiently.
In U.S. Pat. Nos. 4,688,068, 6,147,296, and 6,372,980, though quantum wells used in the multi-junction structure is claimed, the strain in the quantum well still cannot be completely compensated, such that the critical thickness in the crystal growing is thin or lattice defects are easily generated by the mismatching of lattices, which both negatively affect the conversion efficiency of the solar cell finally.